1. Field of the Invention
The invention is in the field of hydrometallurgical treatment of refractory ore for the recovery of precious metal values which are not readily recoverable by the use of ordinary lixiviants. The most common of these refractory ores are the sulfur-containing ores that contain pyrite or arsenopyrite minerals.
2. Description of the Prior Art
With the depletion of reserves of high-grade ores, more interest is being focused on the recovery of gold and silver from "refractory" ores. Refractory ores are those ores from which common lixiviating agents, such as sodium cyanide or thiourea, are unable to leach high yields of a precious metal. The most common of these refractory ores are those that contain pyrite or arsenopyrite as sulfur-containing compounds and such ores are not readily amenable to treatment by leaching. Conventional technology for attempting to recover the precious metals from these refractory ores is first to crush the ores in a series of crushers to obtain a minus one quarter inch (-1/4 inch) product. This -1/4 inch product is then further ground to minus two hundred mesh (-200 mesh) (Tyler Series) and preferably to -270 mesh in order to assure good contact of the ore particles with the treating agent. In the next step, the ground ore is placed in an agitated reactor and treated with an oxidizing agent, such as nitric acid, under conditions of high temperature or high pressure or both. The use of autoclaves enabling pressures of 100 psig to be employed and elevated temperatures of 160.degree. C. is technology known to the industry.
Subsequent to this ore oxidation, the resulting slurry is separated into residual solids containing precious metals such as gold and silver and a liquid fraction which contains solubilized, oxidized metal values. The residual solids then are treated to a standard, conventional extraction with lixiviants such as thiourea, sodium cyanide or other such treatment well known in the art. In this extraction stage, the residual solids from the oxidation stage are placed in an agitated vessel and treated with sodium cyanide solution or thiourea solution to dissolve the gold and silver from the residues of the oxidation step. Once solubilized, the gold and/or silver is recovered by known techniques such as carbon treatment, zinc displacement or the like.
One such patented process is described in U.S. Pat. No. 4,647,307 issued to Raudsepp et al. on Mar. 3, 1987. In this patented process, an ore concentrate containing arsenopyrite or pyrite is finely ground, for example, 60%-200 mesh, and treated in an autoclave to decompose the arsenopyrite in acidic solution in a common volume space through the action of an oxidized nitrogen species in which the nitrogen valence is at least +3. The reaction is carried out at about 80.degree. C. and at elevated pressures. The active oxidized nitrogen species are regenerated in the same common volume space within the autoclave by the injection of oxygen under these super atmospheric pressures. Such oxygen is injected into the autoclave and maintained at a partial pressure of from about 50 psig to about 100 psig. To assure proper mixing of the ore in the autoclave, an agitator is employed to keep the concentrate in suspension and thereby assure good contact of the ore with the treating ingredients in the autoclave. After this reaction has decomposed the arsenopyrite and pyrite, the residual, fine solids can be treated for recovery of precious metals such as gold by conventional lixiviating techniques such as thioureation, cyanidation or the like in agitated treating vessels to assure good contact between the fine solids and lixiviants. In typical examples of the process, the concentrate employed contained about 7 ounces of gold per ton of concentrate and recovery of this gold was substantially increased when the preliminary oxidation step by an oxidized nitrogen specie was carried out.
U.S. Pat. No. 3,793,429 issued to Queneau et al. on Feb. 19, 1974 teaches a preliminary nitric acid treatment of copper sulfide ores and concentrates containing large amounts of copper and iron for recovery of the copper, silver and gold contained in the ore. In this process, the concentrate which contains about 28% copper, 25% iron, 3.5 oz/ton silver and 0.4 oz/ton gold per ton of concentrate is first ground to -270 mesh and subsequently leached with nitric acid at initially 90.degree. C. followed by raising the slurry temperature to boiling for some hours. This action of the nitric acid converts the iron sulfide to hydrogen jarosite or equivalent iron precipitate. The concentrate after being treated by the nitric acid is subjected to a solids liquid separation. The liquid portion is subjected to intermediate purification and neutralization before it is sent to a copper electrowinning stage where copper is recovered. The solids portions which have been separated from the nitric acid leaching stage are treated in intermediate stages for removal of sulfur and unreacted sulfides, such as by froth flotation. Finally, the fine solids slurry is passed to multiple stages of cyanidation, normally carried out in agitated vessels, where the gold and silver are recovered from the insoluble jarosite.
These processes are difficult to carry out because they require high pressure and/or high temperature equipment such as agitated autoclaves and the like and are difficult to operate on a continuous basis and in large scale commercial operations. Further, the grinding or milling of the ore down to 200 mesh or 270 mesh is both time consuming and requires expenditure of large amounts of power. The initial crushing of the ore in stages down to a nominal 1/4 inch to 3/4 inch size is relatively easy and does not require excessive power inputs. However, the grinding and milling of the ore from these nominal sizes down to 200 mesh or 270 mesh requires separate milling operations with high power inputs and specialized grinding equipment. This can be avoided, of course, if the milling of the ore can be eliminated.
Another difficulty is that while the industry is attempting to recover precious metals from low grade refractory ores, the ores which are commonly utilized today contain at least 0.1 ounce of gold per ton of ore in order to assure an economic process. In general, the very low grade refractory gold ores, such as those that contain below 0.1 ounce of gold per ton, many of which contain only 0.05 ounces of gold per ton of ore, are generally too low grade to be processed economically by the known oxidative techniques illustrated in the two patents above, or by other known techniques including roasting or the use of autoclave processing.
Further, these prior art processes do not permit treatment of the chemically oxidized, refractory ores by an economically preferred process for leaching precious metals, namely, heap leaching. The slurry of fine solids obtained after chemical oxidation cannot be stacked in heaps (or beds) which are permeable to treatment with a lixiviant distributed on top of the heap. Washing of such slurry heaps to remove and recover any chemical oxidants and/or lixiviant solutions therefrom is equally impossible because of their impermeability to any washing or treating solutions. In general, heap treatment is normally reserved for treating naturally, not chemically, oxidized siliceous, carbonate-containing ores which are not very highly refractory to lixiviating solutions and thus need no fine grinding and preliminary chemical oxidation.